1. Field of the Invention
The present invention relates generally to methods and apparatus for measurement of very small circuit resistances (0.01 ohm or less) without disconnection of the circuit.
2. Description of the Prior Art
Electromagnetic Pulse (EMP) hardness assurance maintenance and surveillance programs and system life cycle study programs have been established by all branches of the military which have fielded systems with EMP survival requirements. A major portion of these programs is concerned with measurement of cable shield degradation over the lifetime of the system so that short and long term maintenance actions can be planned. To support these programs, several computer based data collection systems featuring network analyzers have been built or are planned. Computer controlled network analyzers provide the capability to collect, process, and store large amounts of high quality data in the form of transfer impedance (Zt) vs frequency plots to 200 MHZ and beyond. These systems emphasize the high frequency response characteristics of the Zt plots to estimate degradation of cable shields. The guiding principle is that holes in the weave of the shield material tend to enlarge as the cable degrades and these cause high frequency resonant peaks to appear in the Zt vs frequency characteristic. Although the use of Zt plots in a 200 MHZ bandwidth allows the data analyst to diagnose cable connector problems accurately and to track gradual deterioration of cable shields over the lifetime of the cable, the method has several drawbacks. First, the cable under test must be disconnected from its equipment and placed in a special cable test fixture. Removal of a cable from its normal environment may eliminate sources of shield degradation which may not be detected in the fixture. The problem may exist in the panel of the equipment from which the cable was removed. Second, the Zt method may yield uninterpretable results at high frequencies when the cable under test is a multi-branch cable. Also, a Zt vs frequency plot may be required for each branch. Third, computer controlled network analyzer systems are very expensive to build and to operate; thus, few systems will be available on short notice and one system may have a heavy case load. Typically, these systems are built into vans or shelters which are periodically transported to sites in the field for scheduled maintenance testing. The time interval between maintenance visits may be as long as a year. Thus, a readily correctable shield flaw may go undetected until the cable can be tested. If the flaw is not with the cable itself, the problem will go undetected despite Zt testing.
The subject inductively coupled low resistance measurement technique is proposed as a very inexpensive complement to the more orthodox method. Since the subject invention can be inexpensively mass produced and can be easily used by relatively unskilled personnel, it may be cost effectively distributed to units in the field. Many of the problems detectable via the subject invention can be repaired on the spot by such actions as cleaning threads on a connector backshell or tightening the coupling nut of a panel connector. With additional effort, the method and apparatus can be adapted to the inspection of conductors in the grounding systems of computer and communications facilities (fixed sites). The method and apparatus could be used to inspect for resistive joints that may develop in ground conductors.
Flaws in the shields of cables can usually be traced to connectors. These flaws which can result from improper bond between cable shield and connector backshell, mechanical stress, or metal oxide buildup at connector junctions, introduce resistances in series with the cable shield and reduce the overall effectiveness of the shield. When such flaws are present, they can be sometimes detected by the above described measurements of transfer impedance vs frequency obtained with a network analyzer or by direct measurement of cable shield resistance obtained with a milliohmeter. The presence of a flaw in the shield will be indicated by an increase in transfer impedance (ohms/length) or shield resistance (ohms) above a previously established maximum allowable value. Typical acceptable values of shield resistance of cables in real systems will range from ten milliohms (0.01 ohm) to several tenths of ohms depending upon such cable parameters as length, diameter, characteristics of the shield material, and allowable junction resistances.
It is not always desirable to attempt detection of cable shield flaws by measurements of transfer impedance or shield resistance by the standard techniques. Both techniques require that the equipment terminating the cable under test be disconnected. When measurements are made on a cable disconnected from its equipment, a serious flaw may go undetected. Disconnection may relieve the mechanical stress that caused the flaw or may eliminate a resistive junction between cable connector and equipment connector. Also, the shield flaw may exist, not within the cable, but at the junction between equipment connector and equipment enclosure. Thus, inspection for flaws in cable shields should be done with the cable connected to its terminating equipment so that all sources of shield degradation will be present in the measurement environment. Therefore, detection of flaws and degradation in cable shields translates to measurement of low valued resistances without disconnecting the circuit under test.
Accordingly, it is an object of the present invention to provide an inductively coupled low resistance measurement method and apparatus which can be used without disconnecting the circuit under test.
It is another object of the present invention to provide an inexpensive cable test set that works in combination with a user supplied portable oscilloscope that provides a continuous display of a pulsed current waveform induced on a cable by the test set.